I'm not sure about your question, but if you're using obj-c, I really recommend Hpple. It's a really good XML/HTML parser.
To use it, you'll need to add ${SDKROOT}/usr/include/libxml2 in "Header Search Path", in your project option and add -lxml2 to "Other Linker Flag". 
Then, when you already have the Hpple files, drag it to your code: TFHpple.h, TFHpple.m, TFHppleElement.h, TFHppleElement.m, XPathQuery.h, XPathQuery.m.
In the code (To get your div "preview"), add:
NSData *htmlData = [[NSString stringWithContentsOfURL:[NSURL URLWithString: @"http://www.yoursite.com/index.html"]] dataUsingEncoding:NSUTF8StringEncoding];
TFHpple *xpathParser = [[TFHpple alloc] initWithHTMLData:htmlData];
NSArray *elements  = [xpathParser searchWithXPathQuery:@"//div[@id='preview']"]; // Here we use 
TFHppleElement *element = [elements objectAtIndex:0];
NSString *string = [element content];
NSLog(@"%@", string);
[xpathParser release];
[htmlData release];
Now we have the "preview div" with Hpple. To get some subclass (as p or a), use it:
NSArray *elements  = [xpathParser searchWithXPathQuery:@"//div[@id='preview']/p/text()"]; 
To undertand more, take a look at XPath Syntax. Also check a tutorial.
Hope it help.